Coalescing agents, or coalescents, are key components in water-borne protective coatings. Glycol ethers, such as ethylene glycol n-butyl ether (EB), diethylene glycol n-butyl ether (DB), propylene glycol monomethyl ether (PM), and dipropylene glycol monomethyl ether (DPM), are commonly used in water-based systems. The glycol ether functions as a coalescent which couples dispersed or emulsified polymer resin particles with water, and helps to keep them dispersed.
Water-borne systems have become increasingly important for environmental reasons, and also because water-borne systems now offer comparable performance to solvent-based systems. Water-borne systems include dispersions and emulsions. Dispersions contain resins such as epoxies, alkyds, polyesters, and acrylics, which can be converted to charged, water-soluble species upon combination with a neutralizing agent. Following application of a water-based dispersion, the water evaporates more quickly than most of the coalescent; the coalescent softens the resin, and allows resin particles to fuse together to form a continuous film. Emulsions contain resins such as acrylic, styrenebutadiene, and polyvinyl acetate. These resins form latex spheres that are emulsified in water with the aid of a surfactant. Upon application, the coalescent evaporates slowly enough from the latex spheres to allow them to coalesce.
Ethylene glycol-derived ethers (E-series ethers) are commonly used in water-borne coatings. Unfortunately, many E-series ethers are toxic, and worker exposure to these chemicals is now regulated. Glycol ethers derived from propylene glycol (P-series ethers), including propylene glycol tert-butyl ether (PTB), offer a low-toxicity alternative to formulators, as described by R. Heckman ("Using P-Series Glycol Ethers in Water-Reducible Coatings," Modern Paint and Coatings, June, 1986, pp. 36-42). Propoxylated PTB is expected to have toxicity characteristics similar to those of PTB.
Ethylene glycol n-butyl ether (EB) is widely used in the coatings industry as a coalescing agent. In addition to its acute toxicity, EB has an undesirable tendency to swell water-borne resins. Unfortunately, the swelling problem is rather unpredictable and varies dramatically with changes in pH. As a result, coating compositions containing EB have viscosities that are highly pH-dependent, especially within pH=8-9, a range commonly employed in formulating water-borne coatings. Coating compositions made with EB are therefore difficult to use.
The acute sensitivity of formulation viscosity to changes in pH when ethylene glycol n-butyl ether (EB) is employed as a coalescent is well illustrated in "RHOPLEX WL Emulsions for Acrylic Lacquers for Industrial Finishing--Formulator's Guide," (Rohm and Haas Company, December 1987) on pages 9 and 10. As shown in FIG. 3E of this reference, the 24-hour equilibrated viscosity of a "RHOPLEX WL-91" water-based formulation with EB as the coalescent increases dramatically from about 1000 cps at pH=7.8 to greater than 10,000 cps at pH=8.4. Because of this steep dependence of viscosity on pH, formulators using conventional coalescents such as EB or EB/DB blends have needed to exercise great care in controlling pH; more flexibility would be highly desirable.
To overcome the pH/viscosity sensitivity problem, formulators commonly increase the level of solvents (typically volatile organic compounds) used in the formulations. Because of increasingly strict emission regulations, coating formulators are being forced to lower the VOC content (increase the percent solids content) of their coatings. Finding coalescents that reduce the viscosity/pH sensitivity of resins would enable formulators to decrease the level of viscosity-reducing solvents needed, thereby reducing VOC emissions and formulation cost.
Some glycol ether coalescents, such as propylene glycol tert-butyl ether (PTB), evaporate too rapidly to allow film formation for some formulations. Others evaporate too slowly, resulting in coatings with poor hardness development profiles.
Another problem in the art is to find suitable coalescents that also have the ability to give coatings with satisfactory minimum film-forming temperatures (MFFT) at low coalescent levels.
It is therefore an object of this invention to provide coalescing agents that have utility in water-based protective coatings, safe handling compared with E-series glycol ethers, viscosities that are relatively insensitive to changes in pH, satisfactory evaporation rates, and good minimum film-forming characteristics.